Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly ("MEA") consisting of a solid polymer electrolyte or ion exchange membrane disposed between two electrodes formed of porous, electrically conductive sheet material, typically carbon fiber paper. The MEA contains a layer of catalyst, typically in the form of finely comminuted platinum, at each membrane/electrode interface to induce the desired electrochemical reaction. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes to an external load.
At the anode, the fuel permeates the porous electrode material and reacts at the catalyst layer to form cations, which migrate through the membrane to the cathode. At the cathode, the oxygen-containing gas supply reacts at the catalyst layer to form anions. The anions formed at the cathode react with the cations to form a reaction product.
In electrochemical fuel cells employing hydrogen as the fuel and oxygen-containing air (or substantially pure oxygen) as the oxidant, the catalyzed reaction at the anode produces hydrogen cations (protons) from the fuel supply. The ion exchange membrane facilitates the migration of hydrogen ions from the anode to the cathode. In addition to conducting hydrogen ions, the membrane isolates the hydrogen-containing fuel stream from the oxygen-containing oxidant stream. At the cathode, oxygen reacts at the catalyst layer to form anions. The anions formed at the cathode react with the hydrogen ions that have crossed the membrane to form liquid water as the reaction product. The anode and cathode reactions in hydrogen/oxygen fuel cells are shown in the following equations:
Anode reaction: EQU H.sub.2 .fwdarw.2H.sup.+ 2e.sup.-
Cathode reaction: EQU 1/2O.sub.2 +2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 O
In typical fuel cells, the MEA is disposed between two electrically conductive plates, each of which has at least one flow passage engraved or milled therein. These fluid flow field plates are typically formed of graphite. The flow passages direct the fuel and oxidant to the respective electrodes, namely, the anode on the fuel side and the cathode on the oxidant side. In a single cell arrangement, fluid flow field plates are provided on each of the anode and cathode sides. The plates act as current collectors, provide support for the electrodes, provide access channels for the fuel and oxidant to the respective anode and cathode surfaces, and provide channels for the removal of water formed during operation of the cell.
Two or more fuel cells can be connected together, generally in series but sometimes in parallel, to increase the overall power output of the assembly. In series arrangements, one side of a given plate serves as an anode plate for one cell and the other side of the plate can serve as the cathode plate for the adjacent cell. Such a series connected multiple fuel cell arrangement is referred to as a fuel cell stack, and is usually held together by tie rods and end plates. The stack typically includes manifolds and inlet ports for directing the fuel (substantially pure hydrogen, methanol reformate or natural gas reformate) and the oxidant (substantially pure oxygen or oxygen-containing air) to the anode and cathode flow field channels. The stack also usually includes a manifold and inlet port for directing the coolant fluid, typically water, to interior channels within the stack to absorb heat generated by the exothermic reaction of hydrogen and oxygen within the fuel cells. The stack also generally includes exhaust manifolds and outlet ports for expelling the unreacted fuel and oxidant gases, each carrying entrained water, as well as an exhaust manifold and outlet port for the coolant water exiting the stack. It is generally convenient to locate all of the inlet and outlet ports at the same end of the stack.
Hydrogen ion conductivity through ion exchange membranes generally requires the presence of water molecules between the surfaces of the membrane. The fuel and oxidant gases are therefore humidified prior to introducing them to the fuel cell to maintain the saturation of the membranes within the MEAs. Ordinarily, the fuel and oxidant gases are humidified by flowing each gas on one side of a water vapor exchange membrane and by flowing deionized water on the opposite side of the membrane. Deionized water is preferred to prevent membrane contamination by undesired ions. In such membrane-based humidification arrangements, water is transferred across the membrane to the fuel and oxidant gases. NAFION is a suitable and convenient humidification membrane material in such applications, but other commercially available water exchange membranes are suitable as well. Other non-membrane based humidification techniques could also be employed, such as exposing the gases directly to water in an evaporation chamber to permit the gas to absorb evaporated water.
It is generally preferred to humidify the fuel and oxidant gases at, or as close as possible to, the operating temperature and pressure of the fuel cell. The ability of gases such as air to absorb water varies significantly with changes in temperature, especially at low operating pressures. Humidification of the air (oxidant) stream at a temperature significantly below fuel cell operating temperature could result in a humidity level sufficiently low to dehydrate the membrane when the stream is introduced to the cell.
In the oxidant stream that is directed through the cathode layer of a fuel cell, there is generally an inverse relationship between the concentration of oxygen and the water content of the oxidant stream. In this regard, the highest concentration of oxygen will normally be found at the inlet of the oxidant stream to the cathode layer. Assuming that no additional sources of fresh oxygen are introduced to the oxidant stream between the inlet and the outlet, the concentration of oxygen in the oxidant stream will become progressively diminished as the oxygen is consumed by the electrochemical reaction at the cathode. In these circumstances, the lowest concentration of oxygen will be found at the outlet of the oxidant stream from the cathode layer. Conversely, the lowest water content will normally be found at the inlet in the oxidant stream to the cathode. As water is generated by the electrochemical reaction at the cathode, the water content will increase as the oxidant stream is directed to the outlet, where the oxidant stream will have the highest water content. Preferably, the temperature of the oxidant stream should be increased (i.e., a positive temperature gradient should be established) between the oxidant stream inlet and oxidant stream outlet of the cathode layer, since the capacity of a gas stream to absorb water increases as its temperature increases.
In conventional fuel cell stacks, the approach to thermal management is to provide isothermal conditions across the entire active area of the cell. For example, in U.S. Pat. No. 5,230,966, a coolant fluid flow field plate is disclosed which has a rib-cage flow channel configuration in an attempt to more uniformly cool, and thereby provide isothermal conditions, across the central, active area of the cell.
The attempt to create isothermal conditions across the cell active area often leads to the existence of zones having different operating conditions, which can be characterized as follows:
Initial Zone (just downstream from the oxidant stream inlet)--the incoming oxidant stream is generally not saturated with water at the operating temperature of the fuel cell and is capable of absorbing product water, formed at the cathode, without saturating the oxidant stream. Under these conditions, however, water will also evaporate from the membrane electrolyte. This dehydration of the electrolyte increases electrolyte resistance, decreases performance and may decrease electrolyte lifetime. PA1 Intermediate Zone (between the oxidant stream inlet and outlet)--eventually, the water content in the oxidant stream will rise to the point where, at the fuel cell operating temperature, the oxidant stream relative humidity will be sufficient to remove the product water without dehydrating the membrane. In this preferred situation, some of the water may be removed in the form of entrained liquid water droplets. PA1 Final Zone (just upstream from the oxidant stream outlet)--the product water generated by the fuel cell has exceeded the water carrying capacity of the oxidant stream at the fixed operating temperature of the fuel cell. No additional product water can therefore be absorbed into the oxidant stream. As a consequence, liquid water may accumulate, resulting in diminished localized fuel cell performance due to impeding access of the oxidant stream to the active electrocatalytic sites at the membrane/electrode interface, and/or the formation of blockages which inhibit the flow of the oxidant stream through some channels in a multiple channel flow field (i.e., flooding). PA1 Initial Zone--decreasing the temperature in this zone results in higher relative humidity to adequately hydrate the membrane, thereby improving localized performance and extending the life of the membrane by avoiding drying out of the membrane; PA1 Intermediate Zone--no change in temperature is sought in this zone; PA1 Final Zone--increasing the temperature in this zone causes more water to become absorbed into the oxidant stream, and to be carried out of the cell via the oxidant stream outlet, without causing flooding and/or difficulties in accessing the membrane/electrode interface. PA1 1. an anode layer comprising a fuel stream inlet and means for flowing within the anode layer a fuel stream introduced at the fuel stream inlet, the fuel stream comprising hydrogen; PA1 2. a cathode layer comprising an oxidant stream inlet, an oxidant stream outlet, and means for flowing a oxidant stream from the oxidant stream inlet to the oxidant stream outlet, the oxidant stream comprising oxygen and water formed by the electrochemical reaction of the hydrogen and the oxygen; and PA1 3. an electrolyte interposed between the anode layer and the cathode layer; and PA1 a. an electrically conductive, substantially fluid impermeable cathode fluid flow field plate having formed therein, on the surface thereof facing the electrolyte, the oxidant stream inlet, the oxidant stream outlet, and the at least one oxidant stream channel; and PA1 b. a sheet of porous electrically conductive material interposed between the cathode fluid flow field plate and the electrolyte, the porous material sheet having a quantity of electrocatalyst deposited on the surface thereof facing the electrolyte. PA1 c. an electrically conductive, substantially fluid impermeable coolant fluid flow field plate having formed therein the coolant stream inlet, the coolant stream outlet, and the at least one coolant stream channel. PA1 a. an electrically conductive, substantially fluid impermeable cathode separator plate; and PA1 b. a sheet of porous electrically conductive material interposed between the cathode separator plate and the electrolyte, the porous material sheet having a quantity of electrocatalyst deposited on the surface thereof facing the electrolyte, the porous material sheet having formed therein, on the surface thereof facing the cathode separator plate, the oxidant stream inlet, the oxidant stream outlet, and the at least one oxidant stream channel. PA1 1. an anode layer comprising at least one fuel stream inlet and means for flowing within the anode layer a fuel stream introduced at the at least one fuel stream inlet; PA1 2. a cathode layer comprising at least one oxidant stream inlet, at least one oxidant stream outlet, and means for flowing an oxidant stream from the at least one oxidant stream inlet to the at least one oxidant stream outlet, the oxidant stream comprising oxygen; PA1 3. an electrolyte interposed between the anode layer and the cathode layer; and PA1 1. a first electrode layer comprising at least one first reactant stream inlet and means for flowing within the first electrode layer a first reactant stream introduced at the at least one first reactant stream inlet; PA1 2. a second electrode layer comprising at least one second reactant stream inlet, at least one second reactant stream outlet, and means for flowing a second reactant stream from the at least one second reactant stream inlet to the at least one second reactant stream outlet, the second reactant stream comprising water formed by the electrochemical reaction of the first reactant and the second reactant; PA1 flowing a coolant stream adjacent the electrode layer such that the coolest region of the cooling layer substantially coincides with the region of the electrode layer in which the reactant stream has the lowest water content and the warmest region of the cooling layer substantially coincides with the region of the electrode layer in which the reactant stream has the highest water content. PA1 controlling the mass flow rate of the adjacent coolant stream such that a temperature gradient is induced in the coolant stream between the coolant stream inlet and the coolant stream outlet. PA1 flowing a coolant stream adjacent the cathode layer such that the coolest region of the cooling layer substantially coincides with the region of the cathode layer in which the oxidant stream has the highest concentration of oxygen and the warmest region of the cooling layer substantially coincides with the region of the cathode layer in which the oxidant stream has the lowest concentration of oxygen. PA1 controlling the mass flow rate of the adjacent coolant stream such that a temperature gradient is induced in the coolant stream between the coolant stream inlet and the coolant stream outlet.
In the present, improved electrochemical fuel cell stack, the temperature in each of the Initial, Intermediate and Final Zones is deliberately adjusted independently of the others in order to achieve satisfactory localized performance in all of the zones. In this regard, the following conditions are sought:
In the present, improved fuel cell stack, biased cooling is achieved by controlling the cooling mechanism associated with each fuel cell. Biased cooling is achieved by adjusting the flow path of the coolant stream and by adjusting the mass flow rate of the coolant stream. In particular, a concurrent or co-flow approach is employed, so that the incoming coolant stream, which enters the coolant flow field channels at its coolest temperature, is flowed adjacent the Initial Zone. As the coolant stream reaches the Intermediate Zone, it has been warmed from the absorption of heat from the electrochemical reaction of hydrogen and oxygen, and the coolant stream will continue to absorb heat and increase in temperature as it flows toward the coolant stream outlet.
As the heated coolant stream flows adjacent the Final Zone, where the coolant stream at its hottest temperature, more water is permitted to enter the vapor state and be carried out of the cell without flooding and/or mass transport problems. The magnitude of the temperature difference between the coolant stream inlet and the coolant stream outlet is adjusted by varying the coolant stream flow rate and can be set to produce the optimal aggregate performance of all zones operating in concert.
As indicated above, the coolant stream in the present, improved fuel-cell stack is directed such that the coolest region of each cooling layer substantially coincides with the region of the adjacent cathode layer in which the oxidant stream has the lowest water content. At the same time, the coolant stream is directed such that the warmest region of each cooling layer substantially coincides with the region of the adjacent cathode layer in which the oxidant stream has the highest water content.
The region of the cathode layer in which the oxidant stream has the lowest water content generally corresponds to the region in which the oxidant stream has the highest concentration of oxygen, typically nearest the inlet of the oxidant stream to the cathode layer. Conversely, the region of the cathode layer in which the oxidant stream has the highest water content generally corresponds to the region in which the oxidant stream has the lowest concentration of oxygen, typically nearest the outlet of the oxidant stream from the cathode layer.
Accordingly, it is an object of the present invention to provide an improved electrochemical fuel cell in which the temperature and mass flow rate of the coolant stream are controlled to optimize the management of water along at least a substantial portion of the reactant stream flow path.
It is also an object of the invention to provide an improved electrochemical fuel cell in which the coolant stream is directed with respect to the oxidant stream such that the temperature profile of the coolant stream matches that required to achieve satisfactory water management conditions along at least a substantial portion of the oxidant stream flow path.